(1) Field of the Invention
This invention relates to an inkjet printing apparatus for printing on printing paper by discharging ink droplets from an inkjet head while moving the inkjet head and the printing paper relative to each other, and to a purging method therefor.
(2) Description of the Related Art
An inkjet printing apparatus performs printing by applying ink to printing paper from an inkjet head spaced from the printing paper. With printing resolution becoming higher, a plurality of nozzles of the inkjet head have an increasingly refined structure. This has resulted in an increased possibility of the nozzles getting clogged (non-discharge of ink droplets). Clogging of the nozzles is caused by the ink droplets plugging up the nozzles, or bubbles entering feed paths of the ink droplets. One method of eliminating the clogging consists in what is called purging, which is carried out by attaching a cap to the nozzles of the inkjet head, and placing the inside of the cap in negative pressure, thereby to eliminate the nozzle clogging.
With such purging, while the ink droplets plugging up the nozzles can be removed, it is difficult to remove the bubbles having entered the feed paths. So, “choke suction” has been proposed as a technique for removing also the bubbles having entered the feed paths.
A first apparatus for carrying out this type of choke suction includes a plurality of feed pipes connecting nozzles of an inkjet head and an ink tank, and a choke mechanism disposed upstream of ink supply to each feed pipe of the inkjet head. See Japanese Patent No. 4687063 (paragraph “0029”), for example.
With this first apparatus, after closing the feed pipes with the choke mechanism, the inside of a cap attached to the inkjet head is placed in negative pressure. Thereafter, ink feed pressure is raised at a rapid rate to open the choke mechanism. This discharges the ink at a burst from the feed pipes through the nozzles.
A second apparatus for carrying out choke suction has a plurality of feed pipes connecting an inkjet head and an ink tank, and a diaphragm pump mounted on each feed pipe. See Japanese Unexamined Patent Publication No. 2010-58303 (paragraphs “0086” and “0094” to “0099”), for example.
With this second apparatus, when carrying out a choke cleaning operation, ink is filled into pump chambers of the diaphragm pumps, and then a cap is attached to close nozzles of the inkjet head. The inside of the cap is placed in negative pressure by suction, and the diaphragm pumps are operated to force-feed the ink filling the pump chambers and discharge it from the nozzles. This discharges the ink at a burst from the feed pipes through the nozzles.
However, the conventional examples with such constructions have the following programs.
With the first conventional apparatus, it is necessary to raise the ink feed pressure at a rapid rate in advance of opening the choke mechanism. This poses problems of complicating control and consuming the ink excessively.
The second conventional apparatus requires the diaphragm pumps mounted on the feed pipes, which leads to a complicated construction and a cost increase. It is also necessary to control the diaphragm pumps, resulting in complicated control. Besides, there is again a problem of consuming the ink excessively.